Field of the Invention
The present invention relates to depositing copper-indium-gallium chalcogenide films using aqueous solutions of precursors.
Description of the Prior Art
Quaternary chalcogenide semiconductors of structure CuAxB1-xZ2 (where A, B=In, Ga or Zn, Sn; Z=S, Se, or Te; 0≦x≦1) are among the leading materials candidates under study as absorber layers for conversion of visible and near infrared solar radiation into electricity in photovoltaic devices. These materials offer several important advantages, including composition tunable band gaps for light absorption matched to the solar spectrum, a large knowledge base of their fundamental properties accumulated over decades of research, and photochemical, chemical, and thermal stability, among others. However, achieving sufficient power/energy conversion efficiencies (i.e., >20%) using appropriate materials and systems that can be prepared at low cost remain fundamental barriers to photovoltaic commercialization.
With regard to the latter, a key issue in lowering costs is the ability to prepare high quality materials and films in large quantities using processes amenable for high throughput manufacturing. Although vacuum techniques such as sputtering and co-evaporation offer exquisite control over composition and deposition of semiconductor absorbers, they remain largely more costly batch processing techniques. Consequently, significant efforts are being expended to develop alternative non-vacuum synthetic and film deposition routes based on liquid phase processes compatible with high throughput manufacturing. These include such diverse well-developed technologies as electrodeposition, sol-gel/chalcogenization, reactive solution deposition, interfacial self-assembly, and spincoating, dipcoating, doctor-blading, or ink printing, alone or in combination with subsequent thermal annealing treatments.
One increasingly popular liquid phase dipcoating technology compatible with high throughput manufacturing is layer-by-layer (LbL) deposition. LbL films are formed via alternate exposure of a substrate to separate aqueous solutions or dispersions containing oppositely multi-charged species. A surface charge reversal occurs during substrate treatment with each species, allowing controlled conformal electrostatic deposition of the oppositely-charged material during the next step. For example, polymer multilayer films are readily prepared using solutions of cationic and anionic polyelectrolytes, with film thickness, structure, and morphology controlled by pH, added salt type, ionic strength, polyelectrolyte molecular weight, and/or temperature during the deposition process. Replacement of one or both polyelectrolyte solutions by appropriately charged nanoparticle dispersions permits fabrication of composite materials.
With regards to solar energy applications, the LbL technique has been increasingly exploited for device fabrication, albeit on smaller scales often constrained by the availability of large amounts of the component nanoparticles. For example, Lee et al. have deposited SnO2 nanoparticle/polyallylamine (PAH) multilayers that were sintered to prepare SnO2 films useful for cascadal energy band gap matching in dye sensitized solar cells (DSSCs). (Kim, et al., “Effect of Layer-by-Layer Assembled SnO2 Interfacial Layers in Photovoltaic Properties of Dye-Sensitized Solar Cells,” Langmuir, 28, 10620-10626 (2012)). Furthermore, Ruhlmann et al. have recently fabricated DSSCs via an LbL approach using polyoxometalate and porphyrin dye components. (Ahmed et al., “A molecular photovoltaic system based on Dawson type polyoxometalate and porphyrin formed by layer-by-layer self assembly,” Journal of the Chemical Society-Chemical Communications, 49, 496-498 (2013)). Zotti et al. have prepared photovoltaic cells from LbL multilayers comprising PbSe nanocrystals and polyvinylpyridine, evaluating semiconductor stability and properties using photoelectrochemical and photoconductivity techniques. (Vercelli et al., “Self-Assembled Structures of Semiconductor Nanocrystals and Polymers for Photovoltaics. PbSe Nanocrystal-Polymer LBL Multilayers. Optical, Electrochemical, Photoelectrochemical, and Photoconductive Properties,” Chemistry of Materials, 22, 2001-2009 (2010)). In similar fashion, Nozik et al. have described LbL deposition of Schottky solar cell devices prepared from PbSe nanocrystals and ethanedithiol crosslinkers, with an unsintered device exhibiting 2.1% efficiency. (Luther et al., “Schottky solar cells based on colloidal nanocrystal films,” Nano Letters, 8, 3488-3492 (2008)). More recently, Srestha et al. have reported a photovoltaic device incorporating a multilayer absorber layer prepared via LbL deposition of polyethylenimine (PEI) and polystyrenesulfonate (PSS)-coated copper-indium-gallium selenide (CIGSe) nanoparticles, demonstrating ˜3.5% efficiency for the non-optimized device. (Hemati et al., “Layer-by-Layer Nanoassembly of Copper Indium Gallium Selenium Nanoparticle Films for Solar Cell Applications,” Journal of Nanomaterials 2012: Article No. 512409 (2012)).